1. Field of the Invention
This invention relates to the positioning control of the read-write head in a rotating disk data storage device and, more particularly, to a positioning method and apparatus that reduces hard and soft error rates during data storage disk operation.
2. Discussion of the Related Art
Rotating disk data storage devices are widely used in the art, offering rapid data file access for reading and writing. These rotating data stores include magnetic disk memories using servo-actuator driven magnetic head assemblies to access rotating magnetic platters. They also include the newer optical disk memory employing a laser read-write head assembly to access write-once or read-mostly rotating optical disks. The present state of the art for magnetic data storage disk files can be appreciated by referring to, for instance, Comstock, et al., "Data Storage on Rigid Disks (Chapter 2)", Magnetic Recording, Vol. 2: Computer Data Storage, (Mee, et al., Eds.), McGraw Hill Book Company, New York, 1988.
The method for reading and writing information to the concentric data tracks in a rotating disk file is subject to data errors arising from head tracking errors that occur during data storage to the file and during data retrieval from the file. A method known in the art for reducing both hard (written) and soft (read) errors during writing and reading from a storage device is to define a track centerline and establish limited offtrack regions about each track centerline on the disk. The use of write offtrack limit (WOL) and read offtrack limit (ROL) is known in the art for magnetic and optical electromechanical storage devices. In principle, the offtrack method provides a threshold measure for inhibiting the read or write function of the head assembly. That is, the write function is inhibited (disabled) when the head position exits the inner or outer WOL. By preventing the writing of information by a recording head during an overlap into an adjacent track arising from a large position error, this limit avoids "hard" or nonrecoverable data errors. The WOLs define a "write limit width" for the track. Head position errors greater than half of this write limit width will force a positive write inhibit decision.
Similarly, when the head position exits the read limit region bounded by two predetermined ROLs, the head read function is inhibited to prevent "soft" errors during the read operation.
The problem in the art is that these offtrack limits are static limits that cannot accommodate changes in head-disk dynamics during read and write operations. For instance, the relative velocity between the data storage track and the head assembly is not considered in the read-write inhibit decision. The fixed offtrack limits can lead to reduced data transfer efficiency because of a tendency for the head assembly to bounce back and forth across the limits during track-following operation under some circumstances with the methods known in the art. Repeated read operations needed to correct soft errors can significantly slow data transfer rates.
Practitioners in the art have developed many useful techniques for improving the steady-state track-following operation in a rotating disk storage device. For instance, in U.S. Pat. No. 4,554,652, Maeda et al. disclose an optical information processor that ascertains radial movement of the optical read-write spot with respect to the track centerline using conventional split threshold methods. Maeda et al provide means for inhibiting the optical writing operation in response to optical head position errors beyond a certain Write Offtrack Limit.
In U.S. Pat. No. 4,730,290 Takasago et al. disclose a tracking error detecting circuit for detecting a laser beam position error from the centerline of a data storage track in an optical read-write data storage device. Takasago et al inhibit optical writing after a position error threshold is first exceeded for a first predetermined time and the offtrack condition next continues for a second predetermined time that is longer than the first predetermined time.
In U.S. Pat. No. 4,839,751, Revels discloses a fine positioning scheme for a track-following servo-actuator in a disk file. Revels teaches a two-threshold tracking method where head position errors that violate either a near-boundary or a far-boundary for one of several prescribed time intervals are interpreted to indicate whether the head assembly is "on-center" or not and to logically exclude noise and other error conditions affecting the "on-center" indication. Revels provides for variation in threshold timing intervals to accommodate noise variations but does not consider inhibit decision modification responsive to changes in head position error or velocity.
In U.S. Pat. No. 4,954,907, Takita discloses an improved head positioning control system for track-following in a data recording disk file. Takita teaches the use of a head velocity detector for determining head velocity with respect to the data storage track centerline. He then computes a new servo-actuator input current signal based on changes in incremental head position and velocity and the previous servo-actuator input current. Takita's teachings rest on the assumption that any change in head acceleration that is not reflected in driver current changes must be entirely a result of shock or vibration. His system operates to damp the effects of such unwanted accelerations. Takita does not consider any novel read-write inhibit decisions for reducing hard or soft errors during his novel track-following procedure.
Because of the prevailing practice of using hard threshold read-write inhibit techniques in rotating data storage files, there is a strongly felt need for improved track-following methods that will reduce soft error rates and improve data transfer efficiency during the read and write operations. The related unresolved problems and deficiencies are solved by this invention in the manner described below.